1. Field of the Invention
The present invention relates to an apparatus and method for improving the economics of hydrocarbon production from a producing well. In particular, the present invention relates to an apparatus and method for selectively lifting produced fluid, including produced hydrocarbons and a portion of produced water, to the ground surface and for injecting the remaining produced water, subsurface, in a subterranean well.
2. Related Art
Conventional hydrocarbon production wells have been constructed in subterranean strata that yield both hydrocarbons, such as oil and gas, and an undesired amount of water. These wells are usually lined with heavy steel pipe called "casing" which is cemented in place so that fluids cannot escape or flow along the space between the casing and the well bore wall. In some wells, large amounts of water are produced along with the hydrocarbons from the onset of production. Alternatively, in other wells, relatively large amounts of water can be produced later during the life of the well.
The production of excess water to the ground surface results in associated costs in both the energy to lift, or "produce," as well as the subsequent handling of the excess produced water after it has arrived at the surface. Moreover, the produced water must be disposed of after it has been brought to the ground surface. Surface handling of excess water, in addition, creates risks of environmental pollution from such incidents as broken lines, spills, overflow of tanks, and other occurrences. Accordingly, many oil production fields and wells often rapidly become uneconomic to produce because of excessive water production.
Various apparatuses and methods have been proposed to overcome the problems associated with excess water production and the aforementioned problems associated with lifting, or producing, this water to the ground surface. Several approaches have been used to produce excess water to the ground surface or to avoid producing the excess water to the ground surface by shutting off the water at the entry into the wellbore. Among these means are: installing larger pumps to pump the water to the ground surface; shutting off the water by injecting gels or resins into the formation; and installing mechanical means in the well to interrupt the flow of water into the wellbore. These approaches, however, have not recognized that effective removal of water from oil or gas wells can be accomplished by transferring the accumulated water subsurface to a water-absorbing injection formation.
An evolving approach to the problem of excess water production is to take advantage of the downhole gravity segregation of produced hydrocarbons and produced water in the wellbore. The excess produced water is then conveyed into an injection formation of the subterranean strata while the oil, for example, and a portion of the produced water are produced, or "lifted," to the ground surface. Such an approach has generally been referred to as an "in-situ" injection method. The conveyance downhole of produced water, without having lifted it to the ground surface, can substantially reduce lease operation costs, thereby extending the economic life of entire fields.
Generally, in-situ methods have required the availability of a suitable injection formation, either below or above the production zone, with sufficient permeability to permit injection of the excess water into the injection formation. In addition, these methods have generally been used where the injection pressure gradients of the candidate injection formations were either low or moderate (i.e., less than 0.5 psi per foot of depth). Practical limitations of the existing equipment and prohibitive costs associated with more expensive and complex pumping equipment have usually restricted use of these in-situ methods where a higher injection pressure gradient field has been encountered.
One example of a conventional production apparatus of the in-situ type is a Dual Action Pumping System ("DAPS") that produces oil and a portion of the water from a casing/tubing annulus on the upstroke of the pump, injects water on the downstroke, and uses the gravity segregation of the oil and water within the annulus. Such an apparatus is shown in U.S. Pat. No. 5,497,832, also assigned to an assignee of the present application, the entirety of which is incorporated herein by reference. Tests of this technology in a number of different wells have shown that gravity segregation of oil and water enable a dual-ported, dual-plunger rod pump to selectively lift produced fluids, including produced hydrocarbons and a portion of produced water, while separating and injecting the remaining produced water into an injection zone within the subterranean strata.
The DAPS apparatus, however, does not solve all of the problems associated with excess water production. In order to overcome the often-encountered moderate to high injection pressures of injection zones, the apparatus requires the use of one or more weighted sinker bars placed above the pump to provide the extra force necessary to overcome the injection pressure opposing the downward movement of the pump. This not only requires more expensive lifting equipment above the pump, but also adds to the overall complexity and cost of the pumping system.
In a further example of the conventional in-situ approach, a coupled rod pump is used for separating and producing oil from water in a well, while simultaneously injecting the water into the producing formation or into an injection formation below the producing formation. Such an apparatus is shown in U.S. Pat. No. 5,697,448. The apparatus employs three spaced packers (upper, middle, and lower). An oil pump is located between the upper and middle packers, and a water pump is located between the middle and lower packers. Produced oil and water are accumulated between the upper and middle packers. The oil is delivered through an opening into the oil pump and fills a cylinder associated with the oil pump. Produced water is allowed to drain through additional passages into the water pump cylinder where it accumulates for injection. Selective pumping of the oil on the upstroke of the pump and the water on the downstroke of the pump is effected by a set of check valves associated with both the oil and water pumps. This apparatus, however, does not provide for, nor is it suited for, injecting water into injection formations with excessive injection pressure gradients.
In another example of an in-situ type apparatus, a formation injection tool, mounted to a bottom-hole tubing pump, carries out underground separation and down-bore in-situ transport and injection of the undesired fluids into an injection formation in the production well. Such an apparatus is shown in U.S. Pat. No. 5,425,416. In order to overcome the often-encountered moderate to high injection pressures of injection zones, this apparatus requires the use of one or more sinker bars placed above the pump to provide the extra force necessary to overcome the injection pressure opposing the downward movement of the pump. In instances where shallow injection zones are encountered, this apparatus requires that numerous sinker bars be used above the pump. This not only requires more expensive lifting equipment above the pump, but also adds to the overall complexity and cost of the pumping system.
Thus, there is a need in the art for an apparatus and method that substantially obviates one or more of the limitations and disadvantages of conventional pumping systems. Particularly, there is a need for a system for lifting produced oil and a portion of the produced water to the ground surface, while injecting the remainder of the produced water into an injection formation. There is a particular need for such a system for injection zones having moderate to high injection pressure gradients.